This invention relates to detectors for sensing the presence of halogen compounds in air, such as those present in air because of refrigerant leaks for example. More particularly, this invention relates to those sensors in which a heated alkaline metal compound is employed as a source of ionizable vapors.
There is a large class of chemical compounds containing halogens which are useful and used in a variety of refrigeration equipment including air conditioners and refrigerators found both in industrial and home settings. These compounds may be either aromatic or aliphatic carbon-halogen liquids or gases typically having low boiling points. To guarantee and test for the mechanical integrity of systems containing such compounds, halogen leak detectors have been developed.
In a typical halogen leak detector, a cylinder of a powdered alkaline metal compound is contained within a thin wrapping of platinum foil which serves as a cathode in the detector. This cylindrical cathode is surrounded by a coil of heated platinum wire which serves as an anode in the detector. Also typically present in this structure is a slotted ceramic cylindrical spacer serving both to hold the cathode and to support the platinum anode windings. This structure which is typically approximately 1 inch long is disposed inside a glass, ceramic, or other heat-proof envelope through which the air to be sampled is passed. The anode of the detector is heated to a temperature of approximately 1,000.degree. C. When air containing a halogen compound such as a leaking refrigerant passes through the detector an alkali metal halide in the vapor is thought to be formed which upon drifting to the heated anode, is ionized thereby generating positive ions which migrate back to the cathode which is held at a negative potential. This ion migration results in a small, but detectable, current produced in an external circuit. The increase in current that results in the external circuit depends upon the concentration of the halogen present in the air as measured in parts per million (ppm).
While the exact mechanism under which this current increase occurs is not fully understood, the detector described above functions well and reliably in detecting halogens. However, there are several limitations. In particular, the detector, as described, is not capable of discriminating between different halogens that may be present. This lack of discriminatory ability is due in large part to the fact that there is only a single heating source maintained at a relatively high temperature of 1,000.degree. C. at which almost all halogens react to activate the detector. Thus, halogen compounds present in the air which are relatively stable compounds as well as those compounds which are relatively unstable will activate the detector. In the prior art detector described above, the source, that is the alkali metal compound, is heated by the anode winding which must be kept at a high temperature in order to produce the necessary amount of reaction and ionization. However, while some degree of thermal activation must be provided to the source, it is not necessary that it be heated to the same temperature as the anode.